Many law reviews these days are trying to limit the free availability of the articles they publish, since that gets them more money from WESTLAW, LEXIS, and HeinOnline.
The Virginia Law Review is doing better: It's making all its new articles (from 2004 on) available for free (and google-findable) on its Web site. Law reviews are nonprofit institutions, dedicated to the spread of legal knowledge (and in the process helping educate the students, and get them better credentials). Editors should want to get more readers -- and thus make their hard work yield more benefit -- and not more dollars.
Journals need money to operate, but my sense is that they generally get an adequate amount from a combination of law school subsidies and subscription revenues. (Not a princely amount, but they're not supposed to get a princely amount.) If they can afford it, and I think most journals can, they should follow the Virginia's lead.